Many subterranean well completions require a liner or casing string to be set in a portion of the wellbore. In some wells, such as wells drilled from platforms or "islands," an extended portion of the liner or casing string must be set in a deviated portion of the wellbore i.e., a wellbore portion at an inclined angle to the vertical. The inclined angle in these extended reach wells frequently approaches 90 degrees (from the vertical) and sometimes exceeds 90 degrees. The result is a well bottom laterally offset from the top of the well by a significant distance.
Current state-of-the-art allows drilling of well bores at almost any incline angle, but current well completion methods have experienced problems when setting casing or liner strings in long, highly deviated wells. When drilling, the drill string is typically rotated, thereby reducing drag forces which retard the drill string from sliding into the wellbore or borehole even in highly deviated wells. However, the configuration, diameter, and weight of casing and liner strings (which are typically larger and heavier than typical drill strings) may preclude rotation, e.g., the added weight and size of a liner string can generate significant drag forces and the torsional forces needed to overcome drag can be greater than the torsional strength force limits of the liner string. Torsional strength limitations can be especially severe for slotted liners.
The greater drag and lack of rotation in these wells may cause the casing or liner pipe string to become differentially stuck before reaching the setting depth. If sufficient additional force (up or down) cannot be applied, a stuck string may result in the effective loss of the well. Even if a stuck string is avoided, the forces needed to overcome the high drag may damage the pipe string.
In spite of these extended reach problems, wells having long, nearly horizontal well intervals may be required for fields having limited surface access. Even for fields where surface access is not a problem, long horizontal well sections may be economically desirable. Higher production rates from horizontal instead of vertical wells may be possible from zones where production of unwanted fluids develops from vertically adjacent beds in vertical wells, e.g., coning of water into an oil producing zone.
A flotation method of placing a liner or casing pipe string into a deviated, liquid filled hole is also known to reduce drag-related problems. This method is illustrated in U.S. Pat. Nos. 4,384,616; 4,986,361; and 5,117,915. However, this method requires the wellbore to be fluid filled and a means for trapping and releasing a less dense fluid within the casing or liner string.
Another method useful in short deviated wells is to attach a top weighted segment to the tubular string being run, e.g., using heavy weight drill string attached to the top of the string. The added top weight (in the upper vertical portion of the well) provides more force to drive or push the tubulars into the deviated portion of the well. However, for wells having a long, nearly horizontal portion, the top weight no longer provides added force to push the string once the weight enters the horizontal portion. In fact, the added weight in the horizontal portion only creates more drag. Thus, the top weight method has been generally limited to wells having only a short deviated portion.